THE SOLAR PHOTOVOLTAIC PANEL SIMULATOR

Transcription

1 THE SOLAR PHOTOVOLTAIC PANEL SIMULATOR SPASOJE MIRIĆ, MILOŠ NEDELJKOVIĆ 11 Key words: Analog circuits, Current source inverter, Modeling, Renewable energy, Simulation. In this paper, a simple photovoltaic (PV) simulator is presented. Based on the mathematical model of the PV cell, practical PV device simulator is designed. Without processor and memory, the simulator may generate arbitrary I-V characteristic for different weather conditions. The idea for the realization PV simulator is supported by theory and confirmed with the simulation and laboratory results. 1. INTRODUCTION Renewable energy sources have gained popularity around the world due to its high predictability and availability [1]. PV solar panels can supply different kind of loads. Sometimes, PV solar panels are connected directly to the load. Moreofften, power converter between the panel and the load is used. PV solar panel has highly nonlinear I-V characteristic. In one part of that characteristic it behaves like current source, and in another one like voltage source. Power converters with appropriate control algorithm are used to adjust that kind of characteristic to the needs of the load, and to maximize the efficiency of the module. Because of that, PV systems research are divided into two areas: array physics, design and optimization, and power conversion systems [2]. Conversion systems, like one proposed in [3], have to be tested on PV systems. Installing PV systems can be challenging and expensive. Even more, characteristics depend on illumination and temperature, which cannot be controlled. PV simulator can emulate desired characteristic of solar panel in every part of year, day, independently of illumination and temperature, which is very convenient for lab testing. There are two methods to simulate real PV panel. First, requires use of the device which can simulate sunlight [4, 5]. The second method is to simulate equation of the PV device. Using the second method it is very convenient to build the circuit shown in the Fig. 1, which completely describes equation of PV device University of Belgrade, School of Electrical Engineering, Bulever kralja Aleksandra 73, Belgrade, Serbia, spasojemiric@gmail.com. 1 Rev. Roum. Sci. Techn. Électrotechn. et Énerg., 60, 3, p , Bucarest, 2015

2 274 Spasoje Mirić, Miloš Nedeljković 2 and makes simulation of PV devices much easier. When second mehod is used, it is important to consider the impact of irradiance and temperature [6]. The model of the PV device, used to build the simulator, is single-diode model. The double-diode model is rejected, because the effect of the second diode can be noticed at low irradiation and low voltage [7]. For single-diode model, five parameters has to be determined: photo current, diode saturation current, diode ideality factor, parallel resistance and series resistance. Some of the methods for determining parameters from real PV device data can be used, for example [8], or procedure similar with described procedure in [9]. In Section 2 PV cell mathematic model and scheme are presented and explained. Section 3 describes the practical realization of the simulator. Simulation and laboratory results are presented in Section PV CELL MATHEMATIC MODEL It is very important to understand the difference between PV cell and whole solar panel. The solar panel can have more PV cells, connected in series or parallel, which depends of desired output I-V characteristic. Single-diode model is used in this paper. The PV cell model has ideal part, which is composed of current source and one diode connected in parallel [8]. Fig. 1 Single-diode model of ideal PV cell and practical PV cell with serial and parallel resistance. In Fig. 1, PV cell is shown its ideal part and practical device. Ideal part of PV cell can be described using basic semiconductor theory: qv I = Ipv, cell I0, cell exp 1 akt, (1) where I pv,cell is the current generated by the incident light (it is directly proportional to the sun irradiation), I 0,cell is the reverse saturation or leakage current of the diode, q is the electron charge ( C), and k is the Boltzmann constant ( J/K), T (in Kelvin) is the temperature of the p-n junction, and a is the diode ideality factor [8]. Equation (1) is the equation for one PV cell. PV solar panels can have different number of PV cells connected in series and/or parallel. The equation which determines I-V characteristic of PV solar panel is:

3 3 The solar photovoltaic panel simulator 275 V + RSI V + RSI I = Ipv I0 exp 1, (2) Va t Rp where I pv and I 0 are the PV and saturation currents, respectively of the array and V t = N s kt/q is the thermal voltage of the array with N s cells connected in series, and R s, R p are the series and parallel resistance of the PV device, respectively. Fig. 2 I-V characteristic of the solar panel with typical important points. The characteristic of the solar panel is shown on the Fig. 2, where typical important points of the characteristic are specified. I sc is the short-circuit current and V oc is the open-circuit voltage. MPP is the maximum power point, I mp, V mp are the current and voltage for the MPP, respectively. During electrical design, different number of PV cells are connected in series N s and parallel N p. Detailed procedure of solar panel electrical design is described in [10]. Parameters in (2) depend of N s and N p. If N p PV cells are connected in parallel, then the currents from (2) are: I pv = N p I pv,cell and I 0 = N p I 0,cell [8]. If the larger I sc is needed, more PV cells are connected in parallel. And, when the larger V oc is needed, more PV cells are connected in series. This is very important fact during the realization of solar panel simulator, which is the main subject of this paper. 3. PRACTICAL REALIZATION OF THE SIMULATOR The solar panel simulator realized in this paper is cheap and simple, without processor, memory, etc. It has a current-controlled power converter which is controlled by analog simulator. Current source, diode and current through the resistor R p from the model, shown in the Fig. 1, are the parts of the control topology. The output of the control topology is a current reference for converter. Resistor R s is connected in series with the load resistor. Different I-V characteristics have different parameters in matemathic models, described by (2). Different models mean that different diode characteristics have to be set. The circuit for setting different diode characteristics is shown in the Fig. 3 [11]. To analyze this circuit, Ebers-Moll model

4 276 Spasoje Mirić, Miloš Nedeljković 4 of a bipolar transistor is used [12]. Ebers-Moll model of transistor gives relation between the base-emitter voltage (V BE ) and collector current (I C ): V BE V BE IE = IES exp 1 IES exp, (3) VT VT where V T = V t is thermal voltage and I ES base-emitter reverse saturation current. From the semiconducter theory its very well known the relation between collector and emitter current: I C = αi E, where α is common-base current gain. Fig. 3 Electrical circuit used for arbitrary setting of the diode characteristic. In Fig. 3 there are two transistors Q 1 and Q 2, so there are two collector currents: I C1 and I C2. If we devide these two currents, we will have: I C2 VBE2 V BE1 = exp. (4) IC1 VT From Fig. 3 voltages V BE1 and V BE2 can be found as: and V = V V R V V 2 BE1 B1 E1 in E1 R1+ R2 BE2 B2 E2 E2 E1, (5) V = V V = V = V, (6) where V B and V E are electrical potentials of the transistors pins of the base and emitter, respectively. If we subract Eq. (5) and Eq. (6), we get: V V = R V. (7) 2 BE2 BE1 in R1+ R2

5 5 The solar photovoltaic panel simulator 277 From Fig. 3, collector currents of the Q 1 and Q 2 are equal: I C1 = +15V/1kΩ and I C2 = V out /R out. Now, from Eq. 4 and Eq. 7 we have analytical expression which relates V in and V out from the Fig. 3: V R + 15V R exp 2 in out = out 1kΩ R1+ R2 Vt V, (8) from which can be concluded that the diode ideality factor is a = 1+R 1 /R 2. Thermal voltage, V t in this equation is thermal voltage of the transistors, and it is not multiplied by N s. Exponent in (8) has negative sign, so the input signal has to be inverted. The voltage in (8), V out, is the current reference. So, the value of the diode saturation current from the model fits to the value of expresion R out 15V/1kΩ, and it should be somewhere about 10 8, depends on the type of solar panel. This requires a very low value of the resistor R out. To solve this, small offset voltage (0.6V) is supplied to the V in, and now the value of the saturation current is: I 0.6V R, (9) 3 0 = out exp avt where exp( 0.6V/aV t ) is somewhere about Offset voltage is supplied to the V in through the resistor of 25 kω shown in Fig. 4. Fig. 4 Whole control logic for PV simulator, where block PV cell is circuit for setting diode characteristic from Fig. 3. It is simpler to use the circuit from Fig. 3 only to simulate one PV cell. Thus, V in is then measured voltage divided by the number of series connected PV cells, N s. That division is realized with the resistor R Voc, where its value in kω is equal to the N s, (Fig. 4). The current I pv is set by the two resistors from Fig. 4, R temp and R Isc. With R temp, temperature of PV device is set. With higher values of the R temp, the

6 278 Spasoje Mirić, Miloš Nedeljković 6 higher temperature of the PV device is set. I sc is set with the resistor R Isc. The resistor R p from Fig. 4 is 1,000 times greater than the real value of the parallel resistance. I ref is the reference for the current controlled converter, which has hysteresis current controller. Fig. 5 Whole simulator. The resistor R s is not part of the control topology. Through it passes current I, so R s has to be designed for the curretn I sc of the simulated PV solar panel. Its value is changeable due to the estimated circuit parameters from Fig. 1. The block diagram of the whole system is shown on the Fig. 5. The current source from Fig. 5 (CCS) is a current controlled power converter. It is equivalent of the currents I pv, I d and the current trough the resistor R p from Fig SIMULATION RESULTS The algorithm explained in Section 3 is simulated and compared with the PV equation of the device. Five important parameters of the model, given in Tab. 1, are set with the correct tuning of the resistors in the scheme shown in Fig. 3. The parameters are derived using method in [8]. Table 1 Parameter values Parameter Value I pv 1.5; 1.3; 1 A I 0,n A a 1.3 R p 500 Ω 0.28 Ω R s The values of five the parameters used in PV circuit model for simulation are shown in Table 1. I pv depend on the light and this value in Tab. 1 is chosen arbitrary. I 0,n is the value of the parameter on the nominal temperature. The parameters a, R p and R s are determined using the procedure given in [7]. The simulation is performed in PSpice.

7 7 The solar photovoltaic panel simulator 279 Fig. 6 Simulation results. The curves without dots are the matematical equation of PV device, and the doted ones are the characteristics of the algorithm explained in Section 3. The results of the simulation are shown in the Fig. 6. It can be noted that the two curves are perfectly matched. The curves without dots are the characteristics of the PV matematic model equation, and the doted ones are the characteristics of the PV model algorithm used for the simulation of the solar panel, explained in the Section 3. The simulation is performed for the three values of the short-circuit current: 1.0A, 1.3A and 1.5A. These results are the good base for the practical realization, and good laboratory results. 5. LABORATORY RESULTS Practical realization of the solar panel simulator is shown on the Fig. 7. The usage of the resistors R temp, R Isc and R Voc is explained in the Section 3. The resistor R 2 is the external resistor used for setting diode ideality factor a. This resistor is shown in Fig. 3. Fig. 7 Practical realization of the solar panel simulator.

8 280 Spasoje Mirić, Miloš Nedeljković 8 Five parameters are set, like in the simulation. For different values of the load resistor, voltage and current at the power output of the simulator are measured. The measured results are shown in the Fig. 8. The doted curves are measured, and the curves without dots are simulated curves. A good match of the laboratory and simulation results can be noticed. Fig. 8 Laboratory and simulation results. 6. CONCLUSIONS One way of the realization of the PV simulator is presented in this paper. The idea is covered by the theory and confirmed with the simulation and laboratory results. It is shown how on easy and cheap way, PV simulator can be realized. Arbitrary PV device characteristics can be performed with this simulator. It can be used for different kind of laboratory testing where PV solar panel characteristic is needed. Received on December 6, 2014 REFERENCES 1. Jie Zhao, Jonathan W. Kimball, A Digitally Implemented Photovoltaic Simulator with a Double Current Mode Controller, Applied Power Electronics Conference and Exposition (APEC), Twenty-Seventh Annual IEEE, J.A. Gow, C. D. Manning, Development of a photovoltaic array model for use in power-electronics simulation studies, Electric Power Applications, IEE Proceedings, 146, 2, 2002.

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